Accessing Scala Parser regular expression match data

Question

I wondering if it\'s possible to get the MatchData generated from the matching regular expression in the grammar below.

object DateParser extends JavaTokenParser         


        

Answer 1

No, you can't do this. If you look at the definition of the Parser used when you convert a regex to a Parser, it throws away all context and just returns the full matched string:

http://lampsvn.epfl.ch/trac/scala/browser/scala/tags/R_2_7_7_final/src/library/scala/util/parsing/combinator/RegexParsers.scala?view=markup#L55

You have a couple of other options, though:

• break up your parser into several smaller parsers (for the tokens you actually want to extract)
• define a custom parser that extracts the values you want and returns a domain object instead of a string

The first would look like

val separator = "-" | "/"
  val year = ("""\d{4}"""r) <~ separator
  val month = ("""\d\d"""r) <~ separator
  val day = """\d\d"""r

  val date = ((year?) ~ (month?) ~ day) map {
    case year ~ month ~ day =>
      (year.getOrElse("2009"), month.getOrElse("11"), day)
  }

The <~ means "require these two tokens together, but only give me the result of the first one.

The ~ means "require these two tokens together and tie them together in a pattern-matchable ~ object.

The ? means that the parser is optional and will return an Option.

The .getOrElse bit provides a default value for when the parser didn't define a value.